Car wheel



' BOO June 20, 1933. F A FAHRENWALD 1,915,158

GAR WHEEL Filed July 6, 1929 gaa? Patented .lune 20, 1933 UNITED STATE PATENT ortica FRANK A. FAHRENWALD, OF CHICAGO, ILLINOIS, ASSIGNOR T SOUTHERN WHEEL COMPANY, OF NEW YORK, N. Y., A CORPORATION 0F GEORGIEh CAR 'WHEEL l Application illed July 6, 1929. Serial No. 376,356.

This invention relates to railroad carwheels and has for its object the improvement of the quality of cast-iron wheels whether these be of the low-carbon or highcarbon type and whether such wheels do or do not contain both chilled-portions and un- A chilled-portions in the same unitary casting and further may be employed on cast iron wheels of any: so-called cast-iron composition ranging from high silicon gray iron to lower silicon type such as show white portions if cast against metal chillers or otherwise cooled rapidly.

This invention reacts in a minor way only as respects the graphite constitutent of thekA Fig. 1 illustrating present practice and Fig.v

2 my improved practice with alternatives; Figs. 3 and 4 are copies of illustrative photomicrographs, Fig. 3 of the result of present practice and Fig. 4 that of my improved practice; and Fig. 5 is a sectional view of a standard railway car-wheel. Most car-wheels have for nearly eighty ears been made with what is known as a chilled tread, that is to say, they are cast in a mold having its peripheral portion made of a massive iron ring termed a chiller owing to its sudden cooling effect upon the molten metal; and this metal 1s composed of iron, carbon, and silicon in .such proportions as to produce gray-iron 1n those parts of the mold which are made of sand, but to produce chilled-White-iron in those parts which are subjected to the peculiar local eifect of the Chiller thus producing an intensely hard and wear-resisting tread portion, combined with such strength and easy machineability in other parts as are exhibited by gray-iron. Such wheels exhibit a superlative degree of performance as regards wearresistance but as regards plate-strength the constantly increasing weight and speed of trains and the constantly more vigorous app llcation of the brakes has decreased 'the factor of safety below what could be desired. It is of course, infeasible to make car-wheels all of White-iron because this would preclude all machining, which is at least necessary for the axle-hole, and because whiteiron, while excessively hard, is also weak and brlttle. Due to the desirability of retaining the chilled tread numerous attempts have been made to increase the strength of the plate by changes in its shape and angle and contour but during this entire period no ma]or improvements have been eifected in the quality or general characteristic of carwheel iron. Production practice has been practically standard throughout this entire period so that neither in type of material nor methods of manufacture have anysubstantial changes orimprovements'heen effected for a great many years. Standard practice in the production of chill-tread car-wheels is to employ iron of approximately the following general composition:

Ingredients Percentages Carbon 3.00 to 3.60 Silicon .50 to .70 Phosphorus.- .200 to .500 Sulphur .090 to .150 Manganese... .55 to .80

ess or apparatus which shall be fundamentally incompatible with present day procedure and equipment, since the investment necessary to establish a car-Wheel foundry is Very considerable and it is desirable to preserve existing business conditions. While my improvements are not restricted to use With chilled-tread Wheels, it will be helpful to an understanding of my invention if the present day existing procedure in casting such wheels be first considered.

Car-wheels of the above composition are made by taking molten metal from the regulation type cupola and pouring it into molds in which the portions which form the hub and plate of the wheel and the' inner surface of the tread are made of green sand While the periphery, (the Wearing surface) of both the flange and tread are cast against what is called a Chiller, that is, a heavy-sectioned cast-iron ring finished to the contour of the wheel-tread. The use of-this Chiller accomplishes several things, chief of which is to harden the Wearing portion' of the Wheel by causing the iron to solidify White with a Brinell hardness of 300 to 500 as against only about 150 to 200 for the other portions of the Wheel. Furthermore, the use of this chiller precludes the necessity for finishing the rolling surface of the wheel Which because of lts extreme hardness could only be accomplished by expensive grinding.

After such a ear-wheel has been cast, as soon as the entire Wheel is solidified and in some cases While the heavier hub portion is still partially molten, the cope, (that is, the upper half of the flask or mold-container) is lifted olf together with the Chiller, the pouring gate is broken off While the iron is still Weak and mushy, and as quickly thereafter as the wheel will permit handling without breaking it is removed to a thermally insulated soaking pit. These pitsare merely upright cylindrical chambers formed in a bank of dry kieselguhr or magnesia or sand, and provided with removable insulated covers. Wheels are placed in these pits in groups up to eight or ten and it is normally planned that these Wheels should be placed in these pits at a temperature at least. above the critical temperature of the iron which is about 13250 Fahrenheit. From this point on the practice may vary slightly from one plant to another, in some cases the wheels will be allowed to cool in a first or primary pit to a temperature of 1000o Fahrenheit to 1200 Fahrenheit, and then removed and placed into a socalled secondary pit for still further cooling at a, somewhat faster rate down to a temperature Where they can be taken out and exposed to atmospheric temperatures without danger of cracking. In other plants they are left 1n the first pits until this condition is reached.

Itmight be remarked in passing that this composite Wheel is very susceptible to cracking if chilled rapidly or unevenly throughout any substantial 10W-temperature range and 1f, for instance, one of these Wheels be shaken out of the mold and laid to one Iside to cool normally in the open air it will almost invariably crack when, or even before, it reachesroom temperature or at least upon receiving a minor shock.

It will be evident from the above that this present-type standard heat-treatment is nothing more or less than a dead annealing in which the wheel is allowed to cool from above its critical temperature very slowly through the critical range and from this temperature down practically to atmospheric conditions at a Very slow and uniform rate and a time-temperature curve showing this rate of cooling would be of a general type common to any slow free cooling body.

'lhe curve shown in Fig. l is typical of this cooling condition and is illustrative of curves actually taken from full size cast-iron car-wheels subjected to the cycle of operation above described. At A. the molten metal is poured into the mold Where after a few minutes of irregularity it cools along the usual parabolic curve until the point B is reached at the temperature Where solidification begins. During solidification the teniperature remains nearly constant but from C. Where solidification is complete, the tem- Y perature falls again until point D is reached where the Wheel is removed from the mold and exposed to the air Where it cools rapidly to the point E, Where it is introduced into a pit with several of its fellows at about the same temperature. This decreases its cooling rate as shown by the reduced slope of the line EF, and on arriving at the critical temperature (approximately 1325" Fahrenheit for this composition) the temperature remains substantially constant for a noticeable time after which it again falls in ay long parabolic curve from G' to H where it is removed to the open air,

After car-Wheels have been finished in the above described manner they are passed upon for railroad service by.. either manufacturers or railroad purchasing inspectors who select for test purposes any number of wheels. from a lot and subject them to two tests which are considered as standard by all car-Wheel manufacturers and by all railroads. The first test is known as the drop test in which the Wheel is placed upon a firm support comprising a heavy iron block or anvil which engages the back of the flange of the wheel at three points only, this anvil being further supported by a verymassive concrete base embedded in the ground. The car wheel so supported is-struck'upon its hub portion by a specified Weight (called a tup) dropped from a standard height. For

a 750 lb. wheel this weight is 250 lbs. and the height 12 feet and these vary with the weight of the wheel being tested. The quality of the wheel is gauged by the number of such blows required to first crack the wheel and then further to break it through entirely. T he standard railroad requirement for the 750 lb. size calls for wheels which will withstand three blows before cracking at any portion of the wheel and twelve blows before breaking into two or more pieces.

The second test is the so-called thermal test wherein the wheels are laid Hat in a sand mold which provides an annular space (varying from 2" to 3 wide depending upon the type of car-wheel) entirely surrounding the tread of the wheel, and molten iron (the same as used for making car- \'-.'heels) is poured into this annular space until it iscompletely lilled. This, of course, subjects the tread of the wheel to a very se-` vere high temperature condition and causes it to heat up much .more rapidly than the inner portions of the wheel which can receive heat only by thermal conduction through the tread portion of the wheel.

This test is intended to simulate and even exceed the abuse of wheels in railroad service during which the tread portion may be heated highly and very rapidly by frictiona-l heat generated by heavy brake-shoe application. The quality of the Wheel in this test is measured by the time which elapses after this annular space has been filled with molten iron until the appearance of the lirst crack anywhere in the wheel, usually across the plate or web portion and present railroad requirements call for wheels of the 750 lb.

size which will stand at least live minutes p j before cracking.

It is not usually diilicult to pass these two tests and infact the majority of wheels made by the best companies will withstand as many as twenty blows to crack and thirtylive blows to break under the drop test and seven minutes to crack under the thermal test. However, the repeated and sometimes abnormal application of the brakes to the treads during operation, alternating with repose periods, imposes upon the plate portions a severe radial stress which although very small in range is also very great in force. Indeed the expansion and contraction of metals comes very near to being the irresistible force of the philosopher although exerted through only a very small linear space; but owing to the complete absence of ductility from gray cast-iron the ratio of stress to strain in the plate portion due to this thermal expansion is enormous and being of a reversing and repeating nature may lead to event-ual breakage of the wheel due to fatigue. Hence while accidents due to actual breakage of car-wheels in service are very rare, due to an inspection system which detects growing cracks before the wheel breaks completely, yet the failure of one wheel may entail' great property loss and endanger many lives. Owing to the Very small linear range of the thermal expansionv and contraction it will be seen that a very slight degree of improvement in the plate portion would substantially eliminate failures from this cause. This is what my invention accomplishes.

My invention effects improvements in carwheels of j the cast-iron type primarily through the employment of a new heat-treating cycle which has been proved through intensive tests which have been underway continuously for the past fourteen months. Although this new heating cycle eects the above described improvements in regulation car-wheels it can also be applied with even relatively more valuable results to the new type of cast-iron car-wheel as described in my co-pending applications hereinbefore identifie In order fully and clearly to describe my invention it is necessary to discuss the procedure in terms of heat-treatment with accompanyingcmetallurgical effects. Ordinarily when cast-iron of the above described composition is allowed to solidify and cool it is made up of a vast multitude of very thin, ilakelike, graphite particles embedded in a matrix of carbon steel. After the iron has been poured and has reached a temperature of, say 1700 Fahrenheit, this matrix in which are embedded the flakes of graphite is of non-magnetic gamma iron holding in solution 1.25% or more of carbon. As the temerature drops the carbon dissolving ability @if this austenite, as it is called, decreases until when the critical temperature is reached (at about 13250 Fahrenheit for this composition) it is able to hold only about .85% carbon in actual solution, the balance of the carbon being precipitated out generally in the form of graphite but sometimes also as particles of carbide or cementite. At a point just above this so-called critical temperature the casting consists essentially of free graphite flakes embedded in a matrix of austenite containing approximately .85% carbon in solid solution. if thetemperature of the iron then be decreased gradually to a point below the critical temperature the gamma iron is converted to alpha iron which is magnetic and of a different atomic crystalline arrangement and in which state it has practically no solvent ability for carbon. is the casting passes through the c1 itical range then a strong tendency exists for the formerly dissolved carbon to be thrown out of solution and under the cooling rates mairitained in normal car-wheel production prat'- tice this actually occcurs and the material which was austenite above the critical temperature becomes pearlite below the critical temperature. This pearlite is practically identical with the pearlite of graphite-'free steel of the eutectoid composition, that is, steel containing approximately .85% carbon. Further, in going t-hrough this critical range there occurs perhaps some further breakdown of the combined carbon into graphite carbon which further prolongs the fiaky intermetallic contour of the graphite particles and perhaps further decreases the continuity of the matrix due to this final precipitation of raphite.

rdinary car-wheel-plates are accordingly composed of pearlite ora truly pearlitic matrix in which are embedded a vast multitude of minute, sharp-edged graphite flakes, so closely packed as to aiforda gray color and almost to sever the metallic matrix; and due to the extreme slow cooling of the car-wheels through the critical range this intergraphitic pearlite is more perfect than occurs in any but especially prepared samples of steel or iron employed for other purposes and treated by other methods.

In my invention the procedure for making chill-tread cast-iron car-wheels is identical with the standard practice as described above up to the point where the wheels are shaken out of the sand. According to my process the car-wheels are then introduced into a pit or furnace provided with temperature-producing and temperature-regulating means which permits the entire wheel to cool to a temperature just above the critical range, (either quickly or slowly as may be desired) and then maintaining this temperature long enough to permit the gamma iron matrix to come to a condition of carbon containing equilibrium. That is to say, the wheels are soaked long enough to permit all of the carbon above the saturation percentage in the austenite at the temperature just above the critical to become precipitated. According to my process the temperature of the entire wheel is then very abruptly depressed through the critical range as by suddenly submerging the wheel in-a pot of molten lead or by any cooling means which is rapid enough to prevent the usual conversion of austenite to pearlite. This step then produces alpha iron which retains in dispersed suspension all of the carbon which has been held in actual chemical solution in the austenite, and the extremely dispersed condition of the carbon or carbides produced by this quenching procedure yields a metallic matrix far stiffer and stronger than would be the case if the normal and customary pearlite vor pcarlitic matrix had been permitted to form.

If wheels which have been subjected to this first step in myivprocess are allowed to cool to room temperature or brought to too lowk temperature in the quenching operation fracture of both the tread and body portion of the wheel is very -likely to occur, often ,with an explosive effect which shatters the wheel into many pieces. It is largely for this reason that in the performance of my invention thus far I have preferably used molten lead as a quenching medium, because if this lead is maintained at a temperature above 650 Fahrenheit, and preferably even above 800o Fahrenheit or 900o Fahrenheit, the quench ing effect is amply rapid while the overall cooling temperature is not suicient to reduce the wheel to a temperature where it becomes liable to crack due to internal stresses. I do not limit myself to this mode of quenching, however,` in case the described precautions are observed. Y g

The next vital and immediate following step in my process is to hold the entire wheel at a temperature just below its critical range; or to bring the wheel up to this temperature and then hold it in case the quenching operation has cooled it below the Vtemperature required for the second step in this treatment. In this second treatment it is necessary to observe two precautions, first, not to permit any portion of the wheel to rise in temperature above the critical temperature or to become hot enough to permit substantial resolution of the carbon contained inthe matrix with resulting formation of pearlite upon subsequently cooling and second, to assure the maintainance of a sufficiently high temperature to impart sufficient mobility to the dispersed carbonl or carbides in the matrix to permit their agglomeration into larger masses. In fact, this second heating is preferably as near the critical temperature as can safely be maintained without accidentally reaching it, and the agglomeration of carbides actually can be carried to a point where they appear as relatively large spheroids of carbide scattered uniformly throughout the matrix. f

lFigure 2 of the drawing shows the timetemperature relationship of my process, and also indicates approximately the time and the temperature areas within which my process has been found'to work with greatest facility. At A as before the molten metal is poured into the mold where after a few minutes of irregularity it cools to the point B where solidification begins. This is complete at the' point C from which temperature the casting cools to the point D where the wheel is removed from the mold and eX- posed to the air where it cools rapidly as in the established procedure. This has the advantage of leaving unchanged the established composition and foundry procedures up to this point. The critical temperature for metal of the composition herein assumed is taken at 13250 Fahrenheit, and according to my preferred procedure the temperature of the wheel is allowed to drop as rapidly as may be to the point E which is taken at ner-.aree

molten lead which reduces all of the wheel to a temperature at least as low as the point G although some parts of the metal may be reduced to thepoint ll or thereabout.

The wheel is then immediately transferred E@ to another temperature regulated chamber and subjected for say four hours to a temperature of l300 Fahrenheit whereby the point l is reached. rlhose portions of the wheel which by the quenching were cooled only to a temperature of 13000 Fahrenheit will accordingly conform to the horizontal continuous line portion of the curve, while those other portions, which by reason of their smaller thickness or greater exposure to the quenching bath were brought to a lower temperature, will rise gradually until the maintained temperature is acquired. it the end of this heat-treatment the wheel is from the furnace and allowed to cool to room temperature, either in the openair or preferably in the usual soaking pit as shown in lig. 2 where d indicates the discontinuity due to the removal'from the pit. rThis pitsoaking is particularly desirable in the case of chilled-tread wheels for which my process has been primarily designedf although as heretofore pointed out it is not restricted thereto. Y

Considerable variations in the temperatures are permissible. rlhus instead of allowing the temperature to fall to the point E, and then to be maintained unchanged for a period, l have found it fairly satisfactory to allow the temperature to fall to the point M corresponding to a temperature of l500 Fahrenheit and thence quenching" abruptly as hereinbefore described. ylhis is equivalent to say that the first heating chamber can sometimes be omitted. lt is desirable however to cool the wheels as close to the critical temperature as possible before quenching since this reduces to a minimum the amount of carbon in solution and hence reduces the amount of carbide which must be taken care of. rlhis first chamber is also desirable to secure uniformity of treatment both as between di'erent wheels and as between dilferent parts of the same wheel,

n which would otherwise be very likely to reach 5 the quenching bath at di'erent temperatures. rlhe use of the first chamber also overcomes any difculty which might otherwise occur from some portion of the wheel having dropped below the crit-ical temperature as indicated at N in Fig. 2.

llt is possible also to maintain the quench ing bath at any temperature above 600 Fahrenheit, assuring a quenching medium which is molten at this temperature and to 5 allow more or less of the wheel actually to reach this temperature, as indicated in Fig. 2; and while l have suggested 13000 Fahrenheit as a desirable reheating temperature, there are many plants which cannot maintain so close a control of their furnace operation as this temperature requires, and it is entirely satisfactory to maintain a temperature of 1275o Fahrenheit, or in fact any temperature above about 900o Fahrenheit as shown at l? in Fig. 2; it should be noted, however, that in case this lower temperature is to be employed it is best to increase the time of treatment since the agm glomeration of the carbides proceeds at a distinctly slower rate at this lower temperature.

A concrete application of my invention may be described briefly as follows: a carwheel after shaking out of the sand and while stilly at a temperature above i400o Fahrenheit is placed in a temperature regulated chamber and held there one hour at l350 l"ahrenheit.4 lt is then picked up by mechanical means, usually by an overhead traveling crane with suspended tongs, and submerged for a period of four minutes in a vat of molten lead which is maintained at a temperature of about 800 Fahrenheit. lin this four minute period of submerging the wheel has not of course uniformly reached the temperature of the molten lead but every portion of the wheel has been brought rapidly through the critical temperature range. After this four minute treatment the wheel is moved into another temperature regulated chamber and maintained four hours at 12950 Fahrenheit, after which it is allowed to cool normally to room temperature either in a soaking pit or even out in the open air.

lt will thus be seen that my time-tempera-s ture treating cycle is very di'erent from that heretofore employed, although the performance of my invention does not necessitate any change in the composition or in methods of melting or molding, or in the casting equipment. 0n an actual test certain wheels treated by my new process required under the drop-test from 40 to 90 blows to crack, and up to 100 blows to break which was between two and three times as many blows as were withstood by an untreated wheel; under the thermal test the wheels which had been subjected to my improved heat-treatment withstood cracking for more than eight minutes, and l have never yet known one of them to break, regardless of the time that the hot testingring may be left in contact with the periphery of the wheel. Furthermore, when em.- ployed with a chilled wheel, no softening or impairment of the tread is experienced.

In Figs. 3 and 4 of the drawing I have illustrated in a measure the remarkable and profound changes which this new heattreatment produces in the iron of the wheel. .lign 3 is copied from a photomicrograph of iron taken from the body portion of-a car-wheel subjected to the standard practice heretofore followed and Fig. 4 is a copy of a photomicrograph of equal magnification of iron of identical composition taken' from an identical position in a wheel produced according to my new process. It will be recognized by those skilled in the art'that- Fig. 3 shows the regulation pearlite matrix while. Fig. 4 shows a matrix entirely devoid of pearlite and comprising carbide-spheroids embedded in a ferrite matrix. The longer the .line G I and the more closely it approaches 13250 Fahrenheit the larger the individual carbide. particles will become. Due to the peculiar requirement of ear-wheels advantageous results are secured with carbide particles so small as to be almost invisible with a microscope, though longer treatment yields proportionally better results. Each substantial region of the two wheels exhibits the-same ultimate chemical analysis; and furthermore the matrix portion in the two cases would show practically the same amount of combined carbon under lchemical analysis, but with this difference, namely, that whereas the pearlitc structure is characterized by the occurrence of this carbide in the form of a .multitude 'of minute, narrow, sharp-edged plates which almost destroy the continuity of the metallic phase, the heat-treatment which I have described has compelled these carbides to become collected in nodular form with a substantial increase in the tenacity of the con"- tinuous phase. j

It will be understood, however, that the microscopic appearance which I have attempted to illustrate in my drawing need not be taken as the essential criterion of my ihvention, especially since the same may be, somewhat modified by differences of heattreatment within the limits I have described, and will certainly be changed by variations in composition well within the limits known in the iron foundry art such for example as flow from the use of small amounts of mauganese, nickel, chromium, etc. It will also be understood that the treatment I have described is not limited to cast-iron in which i the carbon content has been allowedI t0 become precipitated in the form of graphite flakes but may also be applied to cast-iron car-wheels in which the graphite has been induced to collect into nodular masses as described in A my co-pending applications hereinbefore referred to. Indeedby a combination of these two processes, namely Ythat of agglomerating the graphite into nodular form as described in my companion applications and afterward agglomerating into nodular form the cementite content of the matrix portion, a car-wheel of superlative strength can be produced.

On the other hand, it is suflicient for many purposes to subject the wheels to only a small part of the total treatment which could be employed beneficially under my process. Cast-iron wheels when carefully made and inspected under hitherto standard procedures 'are distinctly good wheels and can be relied upon to serve. some scores of thousands of miles. On the. other hand one limitingV factor in theirlife is t-he toughness of the plate as compared with radial expansion and contractions of almost microscopic extent, and a very small increase in elasticity in this plate portion is often sufiicient to afford the desirable additional factor of safety and, in-

deed, to multiply the avera e life several fold. Heretofore in this specification I have described in detail the very best mode of performing my invention, with the object of enabling those skilled in the art to secure a superlative product; but for ordinary pur' poses several of the described refinements can be omitted. A

For example, while it may be theoretically desirable to make the entire Wheel (except the tread) uniform and homogeneous in physical structure, the varying thicknesses of usual flake formation characteristic of theeuteetic alloys such as pearlite. If allowed to' cool slowly through the critical range as shown in Fig. 1, the characteristic pearlite arrangement will be produced and no feasible amount of subsequent heating at lower temperatures can modify it e'ectively. Only by compelling a cooling so quickly that the horizontal section FG of the curve in Fig. l is substantially obliterated can this flake formation be prevented; although all things are relative and the disadvantage of a slower quenching can to some extent be overcome by a longer subsequent rehcating. Quenching in a bath of molten metal of lower melt-ing point is the very best mode of effecting this cooling owing to the high heat conductivity of such a bath, though even this is not able to prevent some degree of euteetie layering in the interior of the comparatively massive hub and tread portions. On the other hand there is no special need for additional strength in these portions but only in the plate which is comparatively thin and hence can be cooled rapidly more easily than the heavy sections. Hence other procedures than a metal bath can be employed for the quenching step. Indeed the qualities of carwheels are notably improved by allowing the unrestrained cooling of the same from the shake-out temperature to a temperature below the critical temperature, followed by a long continued holding just below the critical temperature. Inasmuch as the invariable contemporary practice is to cool them through the critical range under insulated conditions this rapid air cooling can be considered, comparatively speaking as a quenching". The result of such air-quenching varies with different parts of the wheel, the thin parts mostly showing a diffused condition of the cementite or carbon-ingredient which can readily be agglomerated, while the thicker parts show a more pronouncedly lamellar structure characteristic of pearlite and less easily spheroidized.

However the plate portion can be caused in this way to become mostly non-pearlitic. It is desirable that the temperature between quenching and reheating be not allowed to go below about 6000 Fahrenheit and it is 1mportant that after quenching the temperature must not be allowed to rise above the critical lest austenite be reformed and the effect of the quenching obliterated. Hence the second heating chamber is imperative while the first can be dispensed with, although by its use an improved and more uniform product is obtainable.

lt will hence be understood that many changes in detail can be made with out de parting from the scope of my inventive idea and that I do not limit myself in any w1se except as specifically recited in my several claims which I desire may be .construed broadly each independently of hmitations contained inl other claims.

Having thus described my invention what I claim is:

1. The process of making cast-iron carwheels having chilled treads and gray plateportions which contains the steps of quenching the car-wheels quickly from a temperature above about 13500 Fahrenheit (but not substantially greater than 15000 Fahrenheit) to a temperature below about 13000 Fahrenheit (but not substantially below about 6000 Fahrenheit) and thereafter subjecting said car-wheels to a temperature greaterth-an 9000 Fahrenheit but less than 13250 Fahrenheit for a sufficient lengthV of time to enable coalescenee of the carbide content of the matrix portion of the metal at least partially into nodular form.

2. The process of making cast-iron carwheels having chilled treads and gray plateportions which contains the steps of quenching the car-wheels quickly through the critical temperature, and thereafter maintaining them close below but never equal to the critical temperature for a sufficient period of time to permit the coalescence of carbon or carbide particles in the matrix into larger masses, and finally allowing them to cool.

3. The process of making cast-iron carwheels having chilled treads and gray plateportions which contains the steps of casting the same of a metal the continuous phase of which at temperatures above the critical consists of austenite, maintaining the same at a temperature between about 13500 and 14500 Fahrenheit for a sufficient length of time-to permit the amount of carbon dissolved in the austenite to fall to that quantity which'` is normal lfor that temperature, quenching through the critical temperature but not below about 6000 Fahrenheit, maintaining said wheels after quenching and before cooling at a temperature between about 11000 and about 13200 Fahrenheit until the carbon content vhas become agglomer-ated to the desired extent, and finally cooling gradually.

4. The process of heat-treating cast-ir'on car-wheels having chilled treads and gray plate-portions which contains the steps of first maintaining the casting at a temperature just above the critical for a sufficient period of time to convert the metallic phase of the cast-iron to a composition which is not more than slightly hyper-eutectoid, quenching through the critical temperature, and subsequently maintaining the wheels for a sufficient period of time at a temperature just below the critical to permit carbon or carbide particles to coalesce into larger masses.

5. The process of making cast-iron carwheels, which comprises casting the wheel, quenching the cast wheel to a temperature below the critical point with sufficient rapidity to prevent the formation of a substantial amount of pearlite, maintaining the wheel at a temperature just below the critical for a sufficient period of time to permit carbon or carbide particles in the matrix to coalesce into particles having larger masses, and finally cooling gradually to room temperature.

" 6. The process of making cast-iron carwheels, which comprises casting the wheel, cooling the cast wheel to a temperature slightly above the critical and maintaining itat such a temperature for a sufficient length of time to permit the amount of carbon dissolved in the austenite to attain an equilibrium, 'quenching to a point just below the critical temperature, and maintaining the casting at such a temperature for a sufiicient period of time to permit the carbon or carbide particles in the matrix to coalesce into particles having a nodular form, and finally cooling gradually to room temperature.

signature.

FRANK A. FAHRENWALD. 

